This document provides an overview of several drought metrics commonly used across the globe. In this document we focus on:
SNOTEL is an automated system of snowpack and related enviormental sensors. SNOTEL is operated by the Natural Resources Conservation Service (NRCS) of the United States Department of Agriculture in the Western United States. Plots in this module show total snow water equievelant (SWE) and total liquid precipitation (rain + SWE) measured to date compared to historical averages.
The SPI is a commonly used metric which quantifies precipitation anomalies at various timescales. This metric is often used to estimate a range of hydrological conditions that respond to precipation over differing timescales. For example, SPI is related to soil moisture anomalies when calculated over short time scales (days to weeks) but is more related to groundwater and reservoir storage over longer timescales (months to years). The values of SPI can be interpreted as a number of standard deviations away from the average (mean) cumulative precipitation depth for a given time period. The SPI has unique statistical qualities in that it is directly related to precipitation probaility and it can be used to represent both dry (negative values; represented here with warmer colors) and wet (positive values; represented here with cooler colors) conditions.
The SPI quantifies precipitation as a standardized departure from a selected probability distribution function that models the raw precipitation data. The raw precipitation data are typically fitted to a gamma or a Pearson Type III distribution, and then transformed to a normal distribution (Keyantash and NCAR staff, 2018). Normalization of data is important because precipitation data is heavily right hand skewed. This is because smaller precipitation events are much more probable than large events.
Key Strengths:
Key Weaknesses:
There has been extensive validation of the SPI across the globe. In general, results have shown that the SPI provides similar results to different standardized precipitation indices.
https://www.sciencedirect.com/science/article/pii/S0168192318303708
https://link.springer.com/article/10.1007/s10584-005-5358-9
https://www.hydrol-earth-syst-sci.net/17/2359/2013/hess-17-2359-2013.html
https://journals.ametsoc.org/doi/abs/10.1175/JHM-D-13-0190.1
https://journals.ametsoc.org/doi/abs/10.1175/JAMC-D-10-05015.1
Validation in progress
UMRB specific recommendations will be appended to this document as validation is completed
Much of the background information regarding this metric was contributed by NCAR/UCAR Climate Data Guide
SPEI takes into account both precipitation and potential evapotranspiration to describe the wetness/dryness of a time period. Similar to the SPI, SPEI can be calculated at various timescales to represent different drought timescales. As such, the SPEI can approximate different impacts of drought on hydrological conditions and processes depending on the timescale used. Although similar to the SPI, SPEI incorporates the import effect of atmospheric demand on drought which can cause significant impacts over short time scales (flash drought).
SPEI is an extension of the SPI in the sense that it uses a normalized probability distribution approximation of raw values to calculate deviation from normals. Similar to SPI, SPEI values are reported in units of standard deviation or z-score (Vicente-Serrano and NCAR staff, 2015). Although, the raw values for this metric are P-PET.
Key Strengths:
Key Weaknesses:
The SPEI has been used in many studies to understand the effects of drought on hydrologic resource availability, including reservoir, stream discharge and groundwater. In general, SPEI calculated at longer timescales (>12 months) has shown greater correlation with water levels in lakes and reservoirs (McEvoy et al., 2012).
Validation in progress
UMRB specific recommendations will be appended to this document as validation is completed
Much of the background information regarding this metric was adapted from the NCAR/UCAR Climate Data Guide (here)
EDDI calculates the rank of accumulated PET for a given region. Unlike SPI and SPEI, EDDI does not standardize data based off of theoretical (parameterized) probability distributions. Instead, EDDI uses a non-parametric approach to compute empirical probabilities using inverse normal approximation. This method calculates EDDI by ranking the data from smallest to largest and accounting for the number of observations. Therefore, maximum and minimum values of EDDI are constrained by the number of years on record (which determines the number of observations). Practically, this causes EDDI to only show the relative ranking of year, with respect to the period of record, and does not approximate the magnitude of PET anomalies outside of relative ranking.
Key Strengths:
Key Weaknesses:
Validation in progress
UMRB specific recommendations will be appended to this document as validation is completed
Much of the background information regarding this metric was adapted from the NCAR/UCAR Climate Data Guide (here)
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